This invention relates to an improvement in an optical fiber cable having a cabled assembly of optical fiber cores.
A known optical fiber cable is shown in FIG. 3. The cable 35 has a core assembly formed by surrounding a plurality of ribbons of optical fiber cores 31 by a shock absorbing material 32 capable of absorbing water, too, a material 33 of high tensile strength extending along the core assembly, and a sheath 34 formed from e.g. a thermoplastic resin and enclosing the core assembly and the high-tensile material 33. The shock absorbing material 32 protects the cores 31 from external pressure. As it can absorb water, the shock absorbing material 32 absorbs any moisture that may enter the cable with the air passing through the sheath 34 and have an adverse effect on the cores 31 if, for example, the sheath 34 is broken. A strand of fiber of e.g. polyacrylate, or polyvinyl alcohol is usually used as the shock absorbing material 32. It is usually wound about the cores 31 in one or more directions.
The strand is required to be of the lowest possible density to be readily capable of absorbing water and swelling. A strand of low density, however, has a high coefficient of contraction under heat. Its contraction resulting from the high temperature of the cable is likely to cause the excessive tightening of the cores about which it is wound, and thereby bring about an increase of transmission loss.
Under these circumstances, it is an object of this invention to provide an optical fiber cable which can resist any increase of transmission loss caused by temperature variations.
The above objet is essentially attained by an optical fiber cable comprising a core assembly formed by surrounding an optical fiber core or cores by a shock absorbing member, at least one member of high tensile strength extending along the core assembly, and a sheath enclosing the core assembly and the high-tensile member, wherein the shock absorbing member comprises a strand of shock absorbing material not absorbing water, but carrying a water-absorbing resin.
The strand is preferably of yarn not absorbing water. Its principal component may be polypropylene, nylon or carbon yarn. The water-absorbing resin is preferably a water-absorbing polymer bonded to the shock absorbing material by an adhesive. The shock absorbing material preferably has a coefficient of thermal contraction not exceeding 1% when left to stand at a temperature of 130 deg. C. for 30 minutes. The shock absorbing member preferably has a water absorbing power of eight times.
According to a salient feature of this invention, the shock absorbing member is a strand of e.g. yarn not absorbing water, but carrying a water-absorbing resin. The yarn not absorbing water is a material of high density having a low coefficient of thermal contraction. Thus, the shock absorbing member in the optical fiber cable of this invention does not undesirably contract under heat and tighten the core. Therefore, it does not bring about any increase of transmission loss by optical fiber, while it is capable of absorbing water satisfactorily to protect the cable from any moisture.